1. Field of the Invention
The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and more particularly to an apparatus and method for reducing system complexity using an SLM (Selected Mapping) scheme to reduce a PAPR (Peak-to-Average Power Ratio).
2. Description of the Related Art
Conventionally, an OFDM communication system has been defined as an effective digital signal transmission scheme for loading a desired signal to be transmitted on a plurality of sub-band frequencies having carriers that are orthogonal to each other, and transmitting the desired signal loaded on the sub-band frequencies, such that uses an available frequency band at maximum efficiency and can effectively cope with burst errors generable by a fading operation. The OFDM scheme enables a frequency-selective fading phenomenon to approximate a frequency non-selective channel from the viewpoint of individual sub-channels to easily compensate for a serious frequency-selective fading phenomenon using a simple frequency area single-tap equalizer. The OFDM scheme inserts a cyclic prefix that is longer than a length of a multi-path channel delay spread into neighbor symbol blocks to remove interblock interference and interchannel interference, and is appropriate for a high-speed data transmission scheme using an IFFT (Inverse Fast Fourier Transformer) and an FFT (Fast Fourier Transformer).
The sub-band signal used in the OFDM scheme is modulated by the IFFT so that an amplitude of the modulated signal proportional to the number of the sub-bands is displayed in the form of a Gaussian Distribution according to the Central Limit Theorem. Therefore, a transmission signal has disadvantages in that it encounters very high PAPR characteristics along with serious nonlinear distortion, which is worse than that of a single carrier transmission scheme due to nonlinear saturation characteristics of a high-power amplifier used for creating sufficient transmission power in wireless communication environments, resulting in limited performance of the OFDM scheme. Therefore, many developers have conducted intensive research into a variety of solutions for solving the aforementioned problems.
The SLM scheme is a representative solution for reducing the PAPR, which creates U information series independent from each other to indicate the same entry information bit, selects the lowest PAPR among the U information series, and transmits the selected lowest PAPR. The U information series multiply the entry information bit by U mask sequences each having a predetermined length of N, and generate the multiplied result. The SLM scheme abruptly increases the number of calculations needed for an optimum PAPR as the number U of phase series increases, whereas it can maintain a data transfer rate. The SLM scheme uses U IFFTs, which are parallel to each other, to prevent a transmission time from being delayed, resulting in increased complexity of a transmitter.
FIG. 1 is a block diagram illustrating a transmitter for use in an OFDM communication system for use with a conventional SLM scheme. Referring to FIG. 1, an information bit configured in the form of a binary signal is applied to a channel encoder 100 as an input signal. The channel encoder 100 encodes the received information bit to generate coded symbols, and the coded symbols are applied to a mapper 110. The mapper 110 maps the received coded symbols with a single signal contained in a signal constellation. The mapping-processed output signals generated from the mapper 110 collect N signals according to the input magnitude N of the IFFT 140, and form a single signal block. The signal block branches to U branches, and the branched result is applied to a plurality of multipliers 130, 132, and 134. A mask generator 120 generates U independent mask sequences M1, M2, . . . , Mu each having the length of N, and the U mask sequences M1, M2, . . . , Mu are transmitted to the multipliers 130, 132, and 134, respectively.
The multipliers 130, 132, and 134 adapt the signal block and the mask sequences M1, M2, . . . , Mu, as their input signals, respectively. Therefore, the multipliers 130, 132, and 134 perform the multiplication of two input signals, i.e., the signal block and one of the mask sequences M1, M2, . . . , Mu. The output signals of the multipliers 130, 132, and 134 are IFFT-processed by the IFFTs 140, 142, and 144, respectively, such that the IFFTs 140, 142, and 144 output signal sequences S1, S2, . . . , Su, respectively A selector 150 receives the signal sequences S1, S2, . . . , Su, calculates individual PAPRs of the received signal sequences S1, S2, . . . , Su, selects a single signal sequence having the lowest PAPR among the received signal sequences S1, S2, . . . , Su, and transmits the selected signal sequence as a transmission signal.
As described above, the SLM scheme selects a signal block having the lowest PAPR among U signal blocks generated by the same information bit, and transmits the selected signal block in such a way that it can effectively reduce the PAPR. The higher the number U of signal blocks, the lower the PAPR. However, as illustrated in FIG. 1, the SLM uses U parallel IFFTs to prevent a transmission time from being delayed. As a result, the higher the number U of signal blocks, the higher the complexity and cost of production of a transmitter system.